The present invention is directed to a multistage vacuum pump and more specifically to a multistage vacuum pump comprised of a single piston cylinder assembly having an enlarged diameter central portion with one end of the assembly and opposite sides of the enlarged diameter central portion providing three stages interconnected by passage means.
Australian patents numbers 481072 and 516210, which are both assigned to the same Assignee as the present application, disclose various forms of a reciprocatory piston and cylinder machine having a differential piston in two working spaces. In the practical application of such a machine, it is usual to provide multiple cylinders as respective stages of a multistage pump. The machine is particularly well suited for use as a mechanical vacuum pump utilizing solid sealing rings or sleeves in lieu of oil or other liquid lubricants. A four cylinder pump having a pair of parallel coupled high vacuum cylinders, jointly connected in series with a median vacuum cylinder and a low vacuum cylinder is particularly appropriate and has the advantage of being suitable for construction in well-balanced configurations. In prior pumps, the connections between such stages were made by covered passages and external conduits but these are not readily translated into an internal porting and ducting arrangement.
The U.S. Pat. No. 4,560,327 to BEZ et al., discloses a porting and ducting arrangement for a pair of adjacent cylinders of a multistage vacuum pump wherein a plurality of passages extend longitudinally in the walls of the cylinders and communicate with the interiors of the cylinders through respective ports. A plurality of recesses in the form of arcuate depressions may be located in the ends of the cylinder walls or in the bottom surface of the cylinder head which register with respective passages or groups of passages and suitable openings are provided in the cylinder head in communication with the recesses for supplying or exhausting fluid to or from the interiors of the cylinders. This patent is also assigned to the Assignee of the present application.
U.S. Pat. No. 4,790,726 in the name of Balkau et al. and assigned to the same Assignee as the present application, is also directed to a reciprocatory piston and cylinder machine adapted to be used as an oil-free vacuum pump. The vacuum pump disclosed in this patent is directed to a cylinder having a first portion closed at one end and a second portion contiguous with, but of smaller diameter than, the first portion and a piston having a cylinder head portion slidable in the first cylinder portion and a second cylindrical piston portion slidable in the second cylinder portion with said piston head portion having a front face facing the closed cylinder end and an annular back face. A gas inlet is provided for introducing gas to the interior of the first cylinder portion between the front face of the piston head portion and the closed cylinder end on reciprocation of the piston. A first exhaust port is provided for exhausting gas from the interior of the first cylinder portion ahead of the piston head portion by pumping action of the front face of the piston head portion, a one-way valve is provided in the first exhaust port which is operable to permit the exhaust of gas from the interior of the first cylinder portion ahead of the piston head portion and a second exhaust port is provided for the exhaust of gas from the interior of the first cylinder portion behind the piston head portion by the pumping action of the back face of the piston head portion. Sealing means are provided for the piston head portion which includes a sleeve of low friction material disposed on the cylindrical surface of the piston such that over the temperature range encountered during the normal operation of the pump, a mean gap is sustained between the sleeve and the cylinder, which gap is of a maximum size at which leakage of gas past the sleeve is at a level for an acceptable degree of vacuum to be sustained by the pump. A similar sleeve is provided on the second piston portion and resilient means are provided adjacent the end of the sleeve remote from the first piston portion for forcing the sleeve into sliding engagement with the wall of the cylinder. Furthermore, the one way valve in the exhaust port is provided with projecting means which are adapted to be engaged by the piston for opening the valve in the exhaust port controlled thereby on each stroke of the piston even though the pressure within the cylinder is too low to open the valve against the force of the spring biasing the valve into normally closed position.
U.S. Pat. No. 4,854,825 to BEZ et al. and assigned to the same Assignee as the present application, discloses an oil-free, multistage vacuum pump having the cylinders, crankcase and passage means formed in a single casting with two pairs of cylinders opposed to each other in a substantially common plane on opposite sides of the axis of crankshaft support means extending perpendicular to the axes of the cylinders. Each cylinder is provided with a large diameter portion adjacent the cylinder head and a smaller diameter portion adjacent the axis of the crankshaft. A step piston is reciprocally mounted in each sleeve and is operatively connected to a crankshaft for rotation in the crankcase. One pair of piston cylinder assemblies are considered the high pressure pumping assemblies while the other pair of pistons and cylinder assemblies are considered to be the low pressure pumping assemblies. While such a vacuum pump is capable of providing extremely low pressures, the need for the four piston and cylinder assemblies makes the size and weight of the pump much too great and the noise level is too high. Furthermore, such a multistage vacuum pump is very complicated in construction and it requires too many parts and is far too expensive.
One particular feature of the foregoing multistage vacuum pump which presents a great difficulty is the requirement of four atmospheric seals. As the pump requires four pistons for the operation, each piston carries on its small diameter, a flexible seal to prevent leakage of atmospheric pressure air into the working parts of the pump. It would indeed be desirable to prevent any leakage of atmospheric air into or out of the pumps. Not only can air leak into the pump, but exhaust gasses can also leak out of the pump, particularly at the fourth stage of compression where the pressure exceeds atmospheric pressure inside the pump. The same can happen in all cylinders during the high pressure operation, that is, when pumping out a vessel from atmospheric pressure.